Lignin is a three-dimensional macromolecule that occurs in the cell wall of plants and is composed of various phenolic monomer building blocks such as p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. Lignin is incorporated into the plant cell wall during the growth of plants. About 20% to 30% of the dry matter of lignified plants comprises lignin. In addition to cellulose and chitin, lignin is one of the most abundant organic compounds on Earth.
Lignin and lignin-containing substances such as alkali lignin, lignin sulfate, or lignosulfonate occur in large amounts as byproducts in various industrial processes such as paper manufacture. The total production of lignin-containing substances is estimated at about 20 billion tons per year. Lignin is therefore a very valuable raw material. Some of this lignin is used in a number of applications. For example, alkali lignin, which can be produced by alkali treatment of the black liquor arising in paper manufacture, is used in North America as a binder for particle boards based on wood and cellulose, as a dispersant, as an agent for clarification of sugar solutions, and as a stabilizer for asphalt emulsions and foam. The greatest amount of lignin is used to produce energy through combustion.
Because lignin is rich in aromatics, lignin can potentially serve as a source for a number of valuable aromatic polymers, oligomers, and monomers. However, lignin is notoriously difficult to selectively modify or break down in to simpler compounds. Progress in selectively modifying and breaking down lignin into simpler compounds has been made (see U.S. Pat. No. 8,969,534), but further progress on this front is needed.